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HTMLProofer vs W3C Markup Validation Service: catalog facts
HTMLProofer	W3C Markup Validation Service
Free Free and open source.	Free A free service run by the W3C.
Windows, macOS, Linux, Command line	Web
Checks generated output rather than source templates Works well in CI for static sites	Validate by URL, file upload or direct input Choice of character encoding and document type
Requires a Ruby environment Works on local HTML files rather than crawling live sites	HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker
Downloadable app	Web application
Open source	Closed source
License not stated	License not stated
No account needed	No account needed
Not stated if it works offline	Needs internet
Official website Full listing and sources Checked September 23, 2026	Official website Full listing and sources Checked September 23, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.